1. Field of the Invention
The present invention relates to a method of polymerizing a photo-polymerizable composition for dental use, which may be used in filling treatment of dental caries, and also to a beam illuminator for use in filling treatment of the dental caries with the photo-polymerizable composition.
2. Description of the Prior Art
In recent years, the polymerizable dental compositions (composite resins) comprising a polymerizable monomer, a polymerization initiator and an inorganic filler have been improved in physical strength, resistance to wear, color rendering and reproducibility of a tooth morphology and has therefore been used widely not only in dental filling treatment of dental caries and as a material for dental crowns, but also in many other applications. Among others the photo-polymerizable composite resin which initiates polymerization with visible light irradiation has expanded its market because of its ease to use and handle and is now one of the most popular products.
However, the polymerizable composite resin has been found having its own problem, that is, polymerization shrinkage. Specifically, when the photo-polymerizable composite resin undergoes polymerization, a considerably large shrinkage stress develops, resulting in formation of a gap between the eventually polymerized composite resin and the cavity wall. The shrinkage stress is related to a pain that a patient may feel during the dental treatment, and the gap between the polymerized composite resin and the cavity wall often constitutes a cause for loss of the dental filling and generation of secondary caries. Accordingly, attempts have been made to avoid formation of the gap, but have not yet been successful.
With regard to the light-curing apparatus, the currently utilized popular light-curing apparatus comprises a light source installing a halogen-tungsten lamp, a control means including a switch for controlling the light source and a timer for setting the duration of irradiation, a cooling means with a cooling fan, and a light guide means including optical fibers bundled to a diameter of 6 to 16 mm, and is so designed as to irradiate downwardly the photo-polymerizable composite resin filled in the dental cavity with a broad light of a high intensity. Polymerization of the photo-polymerizable composite resin with the above-mentioned light-curing apparatus has been found making it difficult to eliminate the problem of the gap and cracking resulting from the polymerization shrinkage of the photo-polymerizable composite resin.
By way of example, when the photo-polymerizable composite resin filled in a cylindrical dental cavity is irradiated from above by the visible rays of light emitted from the light-curing apparatus, an upper portion of the photo-polymerizable composite resin will be cured upon polymerization to close the cavity surface in a first few seconds. As the irradiation continues, polymerization of the photo-polymerizable composite resin proceeds deep into the dental cavity. The formation of the gap and cracking is, however, found unavoidable because of the polymerization shrinkage. The higher intensity of the light to irradiate, the quicker the photo-polymerization. Accordingly, the quicker the photo-polymerization, the faster the shrinkage of the photo-polymerizable composite resin and, hence, the easier the formation of the gap and cracking.
Also, with the method in which the photo-polymerizable composite resin filled in the dental cavity is irradiated from above the dental cavity, it is impossible for a dentist to ascertain whether or not the photo-polymerizable composite resin in the dental cavity would have been completely polymerized to cure, and it has often occurred that the bottom portion of the photo-polymerizable composite resin in the dental cavity remains unpolymerized. Once this occurs, there is a risk that a monomer component of the unpolymerized composite resin will penetrate in a high concentration into the dental pulp.
In view of the foregoing problems, the Japanese Laid-open Patent Publication No. 64-15037, published in 1989, discloses a shrinkage-compensated differential photo-polymerizing method. According to this publication, two kinds of photo-polymerizable composite resins sensitive to different wavelengths of light were prepared. The first photo-polymerizable composite resin is applied to the wall of the dental cavity, followed by filling of the second photo-polymerizable composite resin. Thereafter, the second photo-polymerizable composite resin is first polymerized by irradiation, followed by photo-polymerization of the first photo-polymerizable composite resin. The method disclosed in the publication is described effective to minimize any possible adverse effect brought about on the teeth by the shrinkage occurring upon polymerization within the cavity.
It appears that the method disclosed in the publication referred to above has the following two features: In the first place, if the second photo-polymerizable composite resin, which is the major filling material, is first polymerized prior to curing of the first photo-polymerizable composite resin which corresponds to a material for protecting the cavity wall and which is used in a relatively small quantity, the polymerization shrinkage occurring in the second photo-polymerizable composite resin can be compensated for by flow or deformation of the first photo-polymerizable composite resin so that shrinkage of the second photo-polymerized composite resin can complete without adversely affecting the teeth.
In the second place, even though the first photo-polymerizable composite resin used in the relatively small quantity, is polymerized subsequently, an averse influence on the teeth which would be brought about by shrinkage of the first photo-polymerizable composite resin would be minimal so long as it is used in a small quantity.
However, not only has the publication referred to above failed to specify particular materials and compositions used for this purpose, but also the processes disclosed therein are indeed complicated, and therefore, the technical effectiveness of the method disclosed therein is questioned. Specifically, considering that the polymerizable dental composite resin is in the form of a slurry or a paste, and even though the cavity wall is protected by the first photo-polymerizable composite resin, an attempt to fill up such a small hole as the caries cavity with the second photo-polymerizable composite resin is quite difficult to accomplish without allowing the first photo-polymerizable composite resin to flow or deform, and the portal edge of the cavity would be more or less covered by the second photo-polymerizable composite resin. In other words, when the second photo-polymerizable composite resin is cured first by polymerization, such composite resin will form a cover over the opening of the dental cavity, making it difficult to avoid developing polymerization shrinkage stresses inside the dental cavity.
In addition, the first and second photo-polymerizable composite resins are described as initiating polymerization upon irradiation with the respective light of different wavelengths. However, the photo-polymerization initiator is generally not sensitive selectively to a certain single wavelength, but sensitive to a relatively broad band of wavelengths. Further in the light-curing apparatus utilizing a halogen lamp, the light emitted therefrom have a broad band of wavelengths. Accordingly, it is difficult to assuredly prevent the first photo-polymerizable composite resin from initiating its polymerization when the second photo-polymerizable composite resin is photo-polymerized and, therefore, it is not avoidable that the second photo-polymerizable composite resin eventually forms a cover over the dental cavity opening.
By the reasons as discussed above, the suggestion disclosed in the publication referred to above appears having numerous problems left unsolved.
On the other hand, the inventors of the present invention have conducted a series of experiments in which with the use of a light dispersive rod (LUCIWEDGE, available from Howe Neos Dental Inc.) a laser beam is introduced into the light dispersive rod through its light source end while a free end of the light dispersive rod opposite to the light source end is inserted into a lump of the composite resin. As a result, the inventors have found that while a substantial amount of the laser beam so introduced had been irradiated outwardly from the whole peripheral surface of the rod, the intensity of the laser beam irradiated was found higher at a location close to the light source end than at any other location remote from the light source end. As regards the extent to which the composite resin is polymerized by the laser beam, the inventors have also confirmed that a head portion of the composite resin was sufficiently cured, but a portion of the composite resin adjacent the free end of the rod was little cured.
Even in the experiments conducted by the inventors in which in place of the light dispersive rod a rod or tube made of transparent plastics was inserted in a lump of the composite resin, a result similar to that described above, that is, in which a portion of the composite resin adjacent the light source end of the rod was much polymerized, but a portion of the composite resin adjacent the free end of the rod was little cured, is obtained. This appears to have resulted from the fact that in the previously described light glide means, a substantial amount of the laser beam was scattered from a portion of the rod adjacent the light source end and the amount of the laser beam scattered from the free end of the rod was small.
Accordingly, with the conventional technique of introducing the laser beam into the composite resin, there is a tendency that only a portion of the composite resin adjacent the opening of the cavity and, hence, adjacent the light source is first cured and the remaining portion of the composite resin adjacent the bottom of the cavity does not cure sufficiently, and accordingly, the problem associated with development of the shrinkage stresses in the cavity wall during the polymerization has not yet been solved.